Isolation and characterization of endophytic Streptomyces strains from surface-sterilized tomato (Lycopersicon esculentum) roots

Aims: To isolate endophytic Streptomyces strains from tomato and examine their antimicrobial activity. Methods: Endophytic Streptomyces strains were isolated using surface-sterilization methods and identified by morphological characteristics. Antimicrobial activities were measured by the agar plate sensitivity method. Antifungal activity in vivo was measured by seedling mortality in infested soils. Results: Twenty-one per cent of endophytic streptomycete isolates produced antibacterial metabolites and 41% produced antifungal metabolites in S medium. Sixty-five per cent of the most frequently isolated strains inhibited the growth of Rhizoctonia solani by the antibiosis assay but only 32% produced metabolites active against R. solani in S medium. Growth promotion and enhanced disease resistance of seedlings inoculated with Streptomyces sp. strain S30 were observed in tomato but not in cucumber seedlings. Conclusions: Endophytic Streptomyces spp. strains were successfully isolated using stringent methods and strain S30 promoted growth and enhanced resistance to R. solani in tomato seedlings. Significance and Impact of the Study: Endophytic streptomycetes showing antifungal activity in vitro and in vivo may indicate the potential for their use as biocontrol agents particularly of R. solani disease of tomato seedlings.